The present invention relates to a steel which is forged into a component a part of which is inductively hardened before use, for example, as constant velocity joints or hub units.
Components, for example, constant velocity joints or hub units, are produced by forming a steel product by cold forging, warm forging, or hot forging or a combination thereof and inductively hardening the forged product particularly in its portion required to have good strength. Steel products, such as JIS S 53 C, SAE 1055, and SAE 1070, are mainly used for such applications.
Due to a recent tendency toward an increase in severity of an environment under which the components are used, or a reduction in size and a reduction in thickness aimed at a reduction in weight, however, further improved rolling resistance level, abrasion resistance, and fatigue strength are required of the conventional quench-hardened portion. In addition, an improvement in fatigue strength in the non-hardened portion, in which the fatigue strength possessed by the as-forged component has hitherto sufficed for the contemplated applications, has also become required.
Further, in these components, there are many sites which undergo machining after casting, an ever-increasing demand in recent years for a reduction in working cost has led to a strong demand for improved machinability.
An increase in the content of carbon (C), silicon (Si), and chromium (Cr) or the addition of molybdenum or the like to improve the properties required of the hardened portion and, at the same time, an increase in fatigue strength of the nonhardened portion by increasing the hardness of the nonhardened portion are considered effective as means for meeting the above demands. Since, however, these components often undergo machining or cold working after forging, unconditionally increasing the hardness of the nonhardened portion is disadvantageous from the viewpoints of machining and cold working. Further, the addition of chromium and molybdenum leads to an increase in material cost. Further, hardening of a portion, which lacks in fatigue strength, is considered effective as means for improving the fatigue strength of the nonhardened portion. This, however, disadvantageously leads to an increase in the number of steps necessary for the production of components which in turn incurs increased production cost. For this reason, meeting a demand for an increase in fatigue strength of the nonhardened portion and a demand for an improvement in properties in the hardened portion while minimizing the increase in the hardness of the as-forged component to ensure machinability and cold workability of the nonhardened portion is required of materials used in these components.
It is an object of the present invention to solve the above problems of the prior art and to provide a steel product which, while minimizing an increase in as-forged hardness to ensure machinability and cold workability, is improved, for example, in fatigue strength in its non-hardened portion and is improved, in its hardened portion, in rolling resistance level, antipitting level, abrasion resistance, and fatigue strength.
First Invention
In order to solve the above object of the present invention, according to the first invention, there is provided a high strength steel for induction hardening comprising, by mass, carbon (C): 0.5 to 0.7%, silicon (Si): 0.5 to 1.0%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, and sulfur (S): not more than 0.035% with the balance consisting of iron (Fe) and unavoidable impurities, the steel being forged into a component a part of which is then inductively hardened before use.
Further, the present invention includes a component, produced by inductively casting at least a part of a product produced by casting the above steel for hardening, for example, a hub unit or a constant velocity joint.
In the above invention, the carbon content and the silicon content are preferably in the range of C: 0.5 to 0.6% and in the range of Si: 0.7 to 0.9%.
According to a preferred embodiment of the first invention, in the high strength steel for induction hardening, the equivalent of carbon Ceq represented by formula (1) satisfies a requirement represented by formula (2):
Ceq=C%+{fraction (1/7)}Si%+⅕Mn%+{fraction (1/9)}Cr%xe2x88x92{fraction (5/7)}S%xe2x80x83xe2x80x83(1) 
0.75xe2x89xa6Ceqxe2x89xa60.90xe2x80x83xe2x80x83(2) 
Second Invention According to the second invention, there is provided a high strength steel for induction hardening, comprising, by mass, carbon (C): 0.5 to 0.7%, silicon (Si): 0.5 to 1.0%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use.
Further, the present invention includes a component, produced by inductively hardening at least a part of a product produced by casting the above steel for hardening, for example, a hub unit or a constant velocity joint.
In the above invention, the carbon content and the silicon content are preferably in the range of C: 0.5 to 0.6% and in the range of Si: 0.7 to 0.9%.
According to a preferred embodiment of the second invention, in the high strength steel for induction hardening, the equivalent of carbon Ceq represented by formula (1) satisfies a requirement represented by formula (2):
Ceq=C%+{fraction (1/7)}Si% +⅕Mn%+{fraction (1/9)}Cr%xe2x88x92{fraction (5/7)}S%+V%xe2x80x83xe2x80x83(1) 
0.75xe2x89xa6Ceqxe2x89xa60.90xe2x80x83xe2x80x83(2) 